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Explainer: what is deep vein thrombosis?

Living in Australia, we’re used to flying long distances. So you’ve probably wondered about the risk of developing a deep vein thrombosis. Perhaps you’ve even considered buying some pressure stockings…

Venous thromboembolism is the fifth leading cause of death in Australia. Image from

Living in Australia, we’re used to flying long distances. So you’ve probably wondered about the risk of developing a deep vein thrombosis. Perhaps you’ve even considered buying some pressure stockings for that next long-haul flight?

So, what is deep vein thrombosis? And what does the evidence say about reducing our risk of developing it?

Deep vein thrombosis is the formation of a blood clot (called a thrombus) in the deep veins of the leg. The clot can be either located in the lower leg or in the thigh, or both. Rarely, a blood clot develops in other veins such as in the arm.

Eventually, the thrombus is in danger of dislocating from the vessels in the leg and going straight into the lung circulation (pulmonary embolism), thus blocking the blood supply of the lung and leading to shortness of breath.

Venous thromboembolism (VTE) – the term which encompasses both deep vein thrombosis and pulmonary embolism – affects around 52 in every 100,000 Australians and is the country’s fifth leading cause of death. So early detection and treatment is vital.

On the other end of the spectrum, blood clots can also form in the more superficial veins of the leg, just under the skin. This is called thrombophlebitis and is a much less serious condition.

Who is at risk?

There are three principal mechanisms that increase the likeliness of developing deep vein thrombosis:

  1. Reduced flow of blood (from being immobilised due to illness, leg injury, or long sitting during long-haul flights),

  2. Increased tendency of blood clotting (due to hereditary diseases such as Factor V Leiden disease)

  3. Injury of blood vessels (from accidents or surgery).

The risk of developing a deep vein thrombosis is increased in patients who have previously had deep vein thrombosis or a pulmonary embolism, and in those with a family history of blood clots.

Other risk factors include cancer (or cancer treatment), taking contraceptive pills containing oestrogen, hormone-replacement therapy, pregnancy and conditions that cause blood clotting, such as thrombophilia.

As some of these risk factors are modifiable, there is a chance to reduce your risk by losing excess weight, quitting smoking (as smoking affects blood clotting and circulation), and using contraception methods other than oestrogen-containing pills.

On long-haul flights, car rides or bus trips, exercise your lower calf muscles. Whenever possible, get up and walk around, or raise and lower the heels while keeping the toes on the floor while sitting.

Get up and walk around as much as possible on long-haul flights. Image from


The first signs of deep vein thrombosis are swelling of the entire leg or, more often, one side of the calf. Sometimes there is a localised painful tenderness and reddening.

In case of the life-threatening complication of lung embolism, the symptoms are sudden shortness of breath with rapid pulse (heart rate), sweating and coughing up blood. If you have any of these symptoms, see your health practitioner immediately.

After a series of questions about the onset and characteristics of your symptoms and a thorough physical examination, further testing will confirm the diagnosis. The best way to diagnose a suspected deep vein thrombosis is an ultrasound examination of the leg. In case of a suspected pulmonary embolism, other special imaging diagnostics (computer tomography or scintigraphy) need to be applied.

Treatment options

The aims of the treatment are to stop the blood clot from getting bigger, from breaking loose – and drifting into the lung leading to pulmonary embolism – and to reduce the chances of deep vein thrombosis happening again.

Deep vein thrombosis is treated with blood thinners (anticoagulants), usually for a period of three to six months. These are mostly administered as injections in the first days, followed by tablets.

Compression stockings of the lower leg prevent the blood from pooling and subsequent clotting. The stockings should be worn for at least one year and after that, whenever immobilised, such as on long-haul flights.

The stockings also prevent one common complication that frequently occurs after deep vein thrombosis: post-thrombotic syndrome, which arises from the damage of the veins caused by the blood clot. The syndrome comprises swelling of the affected leg, pain and skin discolouration.

So, should you have an injection or wear compression stockings when you fly?

Long-haul flights (for more than four hours) increase the risk for developing deep vein thrombosis, like every other condition where your movement is restricted. Although few studies have been performed to address this question, the increase in risk seems small.

It’s important to assess the thrombosis risk on an individual basis. People at the highest risk of travel-related thrombosis who travel more than three hours should wear compression stockings. The stockings need to be individually adjusted to ensure they don’t restrict the blood flow and thereby cause, rather than prevent, thrombosis.

There’s no need to wear compression stockings unless you’re at an increased risk of deep vein thrombosis. Image from

In general, a prophylactic injection of heparin is not recommended and wearing compression stockings on each flight has not been proven to be beneficial. This advice is, of course, different for people, who have had a previous venous thromboembolism or who have more than one risk factor for developing blood clots.

In any case, it’s important you try to reduce the modifiable risk factors for deep vein thrombosis, particularly when travelling long distances.

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6 Comments sorted by

  1. Paul Rogers


    Is aspirin any use as a preventative prior to long flights?

  2. Sophia Avery


    I think it is also important to note that the compression stockings should provide graduated compression to prevent blood pooling in a sitting or mobile patient. I often see entirely mobile patients still wearing Teds(tm) stockings as these were recommended by hospital clinicians (obviously when the patient was hospitalised and lying down for a period of time).

  3. Wayne Gibbens

    logged in via email

    Excellent info, enjoyed the article, and as someone that regularly flies long haul flights it is a topic of interest. Just a question, is swelling feet a symptom of poor circulation that may also have an impact on this? Or is this a symptom of something else entirely? Cheers.

    1. Sue Ieraci

      Public hospital clinician

      In reply to Wayne Gibbens

      Wayne Gibbens - the swelling of feet that occurs on long-haul flights is to do with so-called venous hydrostatic pressure (the pressure of the venous blood in leg veins).

      When standing and walking, the leg muscles provide a "muscle pump" that pushes the blood in the leg veins against gravity back into the chest and into the right side of the heart.

      When sitting, not walking and in an altered pressure environment, the blood pools in the leg veins, pressure in the veins increases, and fluid seeps, under that hydrostatic pressure, into the surrounding tissues. It pools in the feet under the influence of gravity (hence the benefit of keeping the feet raised).

      This is a different mechanism to the ankle swelling of people with heart failure or protein deficiency.